About
What we know about Kigai Yuuto could practically fit in a thimble. He's one of the seven Angels in CLAMP's X, and he's the same in Tsubasa, when Syaoran's group lands in the X world. Most of the following characterization comes from X, where he's a much more regular character; however, from his brief appearance in Tsubasa so far, he seems to be very much the same mysterious character.
He's a watercaster, one of the four characters whose powers are derived from the four elements. He's a government employee who issues marriage licenses.
He has a younger sister named Tomoe. She's afraid of earthquakes.
He's a flirt, and he's compared to a gigolo by other characters. Yuuto rarely seems to be serious; he's always grinning merrily.
All of this combines to form an incomplete picture, a puzzle that has enough pieces to form the beginnings of an image, but not enough to see what that image actually is. I think that's the way Yuuto likes it. When we first meet him, he fights Arisugawa Sorata - with an electric whip, giving Sorata and the audience the entirely wrong impression about the nature of his powers.
In many ways, Yuuto's very much like the element he commands; he's calm on the surface, but something turbulent is happening underneath. He never reveals his true nature. Kigai Yuuto is, in short, the most dangerous kind of trickster - he's the devil you know.